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Since the dawn of sports nutrition, endurance athletes have been relying on high carbohydrate intake to fuel their activity. It’s a technique that has proven to work well for many of the best long-distance performers in the world. However, we’re now starting to discover that it might not be the only way. In fact, it might not even be the optimal way.
In recent years, the low carb diet for endurance athletes has gained prevalence. Its benefits have largely being touted as beneficial in the ultra-endurance scene, but It’s more recently started infiltrating the world of ‘mainstream’ sports science.
In this article, we take a look at the mechanisms and physiology of low carb diets. We discuss how they might improve the performance of endurance athletes, while also giving balance to the conversation by assessing some of the pitfalls. Stay tuned.
What is ‘low carb’?
Low carb is short for ‘low carbohydrate’. It’s a term used to describe a dietary pattern that is lower than average in carbohydrates. There is no set definition on the amount of carbohydrates allowed within a ‘low carb’ diet category, because it’s a loose term.
Generally speaking, a low carb diet could range anywhere from 0-150g of carbs per day. Athletes with a high performance output may see results with 150g of carbs per day, while a sedentary person may need to ingest less than 30g per day if they want the same physiological benefits.
Since the amount of calories ingested from carbohydrates has been reduced, these will then need to be replaced by protein and fat. Most low carb athletes will generally eat a high amount of fat, which can then be burned as a fuel source. A moderate yet sufficient amount of protein is then ingested to ensure muscle recovery.
Why are endurance athletes going low carb?
Endurance athletes are training using a low carb diet in an attempt to become ‘fat adapted’. It’s a term that refers to one’s ability to efficiently burn fat for fuel in the absence of carbohydrate. In essence; once fat adapted, you will no longer require constant carbohydrate intake to maintain energy, but will instead rely on your fat stores to keep you going.
The perceived advantage of being a fat adapted athlete is that your body can seamlessly switch between burning stored body fat or newly ingested carbohydrate. It allows your body to easily access all of the energy it needs at all times, resulting in sustained energy levels over long time periods.
Another reason that endurance athletes are drawn to low carb adaptation centers around food. Ingesting large amounts of food mid-run can often lead to gastrointestinal distress, bloating and nausea. If you require less nutrients during a race, you will negate many of these issues while also reducing your time spent at aid stations.
It’s easy to see why the benefits of fat adaptation has lead so many endurance athletes to tinker with a low carb diet. The mechanisms make a lot of sense, but do we have the data in support of these benefits?
How are endurance athletes becoming fat adapted?
Somebody can be considered ‘fat adapted’ when their body becomes adept at burning fat instead of carbs during endurance activity. Some athletes may take months to achieve true fat adaptation, while others have shown dramatic improvement of fat oxidation after just 2 weeks.
In order to reach a fat adapted state, athletes must first restrict carbohydrates in order for the body to start burning fat. Using this method, most athletes will follow a ‘ketogenic’ diet, which generally allows no more than 50g of carbs per day.
After a few days of following a ketogenic diet, the athlete will then be in a state of ‘ketosis’. This is a starvation mechanism developed by the body, allowing it to survive almost entirely on stored fat in the event of total carbohydrate restriction.
While in a low carb induced state of ketosis, the athlete should continue their training program. Training in this carb depleted state will ensure the body is burning fat for energy, which will then lead to adaptation.
It’s worth noting that fasting is another method by which you can rapidly deplete stored carbohydrate in order to reach a state of ketosis. Many athletes will train in a fasted state to increase the body’s reliance on fat, which will ultimately lead to fat adaptation.
Studies looking at low carb for endurance athletes
In one 2015 analysis, low carb adapted runners were found to have extraordinarily high levels of fat oxidation in comparison to high carb runners. Essentially, they were burning more fat for fuel (surprise!).
Interestingly, the same study found that muscle glycogen (carbohydrate) stores were utilized and restored at a similar amount to high carb athletes. This points to the fact that the body of a fat adapted runner is efficient at converting fat into glycogen, which can then be used for quick-release fuel during high exertion.
A more recent review conducted in 2019 looked to summarize the data around very low carb ‘keto’ diets and how they impact athletic performance. It found limited studies showing a significant improvement in endurance events and stated that high intensity exercise may actually be impaired.
The most in depth analysis so far was released in 2020 by Louise Burke and colleagues from the Australian Institute of Sport. The paper discussed low carb diets and questioned if they were the future of sport.
The paper looked at a range of studies over multiple years, many of which monitored the performance and fuel utilization of elite race walkers. It concluded that periodic low carb dieting during training can result in better fat adaptation, but could also impair the muscles ability to use carbohydrates efficiently.
The authors state that this would especially be an issue during times of higher intensity, while also making the point that individual reactions to the low carb diet can vary dramatically.
Overall, the totality of evidence points to mixed results across the board, with most studies showing an improvement in fat usage but impairment of carbohydrate utilization. In essence, it points to the fact that a low carb diet may be beneficial for endurance athletes, but is most likely detrimental during high intensity activities.
The idea of low carb athletes struggling at higher intensities is also backed up by randomized crossover study conducted in 2019. It found that although low intensity performance output was preserved, high intensity output was diminished in multiple athletes.
Why a low carb diet might not work during high intensity
The method by which fat is oxidized and subsequently burned for fuel requires large amounts of oxygen. In fact, it takes a lot more oxygen to convert fat into fuel than it does for carbohydrate.
In times of low intensity exercise, plenty of oxygen is available to mobilize fat and produce energy. However, in times of higher intensity, oxygen supply is limited and therefore the body finds it more challenging to burn fat.
On this basis, the body will preferentially burn carbohydrate for fuel in times of higher intensity. In general, it appears that activity over 70% of maximum threshold (high intensity) is best performed using a high carb diet.
In fact, previous studies have shown that fat adapted athletes are less efficient at converting stored muscle glycogen (carbs) into fuel even when they load up with carbs before the activity. Another study also showed that converting stored muscle glycogen into energy is often impaired in low carb athletes.
The key takeaway from this? Low carb diets are probably not optimal for higher intensity exercise. Despite being better at utilizing fat during times of high exertion, the data shows low carb athletes may still underperform in comparison to those with a high carb intake.
Is a low carb diet good for endurance athletes?
It depends. As seen in multiple studies, the response to a low carb or ‘keto’ diet can be highly individual. Some individuals tend to retain endurance by burning fat and can even maintain performance at maximal intensity.
The key to determining whether a low carb diet for endurance athletes is optimal will depend on individual goals, body composition and personal experience.
A low carb diet can certainly be good for endurance athletes. It has been shown to significantly increase fat oxidation levels during endurance events that require long-distance low-intensity output.
Some individuals have also seen their metabolic flexibility significantly increase. This allows them to switch between fat and carb burning depending on the situation (e.g. fat burning at slow steady state, carb burning during sprints or hill climbs).
On the flipside, some athletes have seen reduced power output and diminished performance after switching to a low carb diet. Multiple studies in this article have also failed to provide evidence that a low carb diet can improve endurance in comparison to a high carb diet.
The vast range of study results, proposed mechanisms and personal prospective make it clear that each athlete will see different results from low carb training and adaptation. In theory, a low carb diet is good for endurance athletes. However, in reality, the results should always dictate your approach.
Pros and cons of low carb for endurance athletes
- Improved fat burning ability
- Sustained energy source over long distance
- Less nutrition required during performance
- Proven method to reduce bodyweight during training
- Good for low ‘steady state’ cardio
- Potential for improved sense of wellbeing
- Potentially impaired high intensity output
- Limited study evidence pointing to benefits
- Not a tactic that works for all athletes
- Can reduce workout quality during training
- Sub-optimal for high intensity exercise
My experience as a low carb endurance athlete
Based on personal experience, I will say that low carb works for me in specific athletic environments. I have been following a ‘targeted’ low carb diet for a number of years. In this approach, I tend to do most of my training in a fasted state or immediately following a low carb ‘keto’ breakfast. I will then eat my carbohydrates immediately post-workout, which I find helps to expedite recovery.
My experience pretty much follows the scientific data we have. I find that I’m able to perform my long ‘steady state’ runs easily without the need for carbs. I don’t require nutrition during training runs up to marathon distance and beyond. I feel consistent energy during my performance and never ‘hit the wall’ or ‘bonk’.
However, I will note that without pre-workout carbs I notice a reducing in power output during high-intensity training (such as HIIT or repeats) and strength training. This is consistent with the data so should not come as a surprise.
I believe that becoming metabolically flexible through a combination of fasted workouts, carb restriction and targeted carbohydrate intake is the best approach for most athletes. However, finding the right nutrient ratio and timing is a personal decision. You need to find what works for you.
The information in this article provides a well-rounded analysis of low carb for endurance athletes. The mechanisms and theories at hand are relatively well understood, but the effectiveness is still questionable.
Based on the totality of evidence, it’s probably too soon to confirm if a low carb diet is good for endurance athletes. However, it certainly warrants a conversation and probably a few N-of-1 experiments.